As in the case of Nerinœa, so also in this genus, we are dealing with its earliest representatives in so far as this district is concerned, since Tate and Blake make no mention of the occurrence of Alaria in the Yorkshire Lias. Piette, who accepts with modification Morris and Lycett's genus for this section of the winged shells, is of opinion that no truly winged shell has ever been found in the Lower Lias of France, though he describes species from the Middle and Upper Lias, none of which, according to his views, pass upwards. It was in the Lower Oolites that the genus Alaria first began to flourish, and we find in most places that it became tolerably well represented as low down as the Inferior Oolite or Bajocian subdivision.

In determining the actual position of strata from the appearances presented by their exposed edges in natural and artificial sections, certain mathematical problems are of constant occurrence. To a field-geologist who is not content with rough guesses founded on judgments by eye, the solution of these problems is a matter of importance, and methods have accordingly been given for some of those most frequently met with. These solutions take the form of (1.) trigonometrical formulæ, which can be applied only with the aid of trigonometrical and logarithmic tables; (2.) tables specially prepared from these formulæ for use in the field; (3.) graphical methods, requiring only a ruler, scale and protractor, which may be conveniently combined in one instrument. Of the first kind is the formula for deducing the true dip of strata from two apparent dips, given in Green's Geology (p. 341, 1st ed.), etc. Among special tables are those of Mr. Jukes for finding the apparent dip in any direction from the true dip, and for connecting the dip, thickness and depth of beds: these tables are given in the appendix to the Survey Memoir on the Geology of the South Staffordshire Coal-field, and reproduced in Jukes's “Manual of Geology.” Graphical methods have been used for finding the true dip from two apparent dips: a method partly graphical but requiring a table of cotangents is given in Phillips's “Treatise on Geology” (p. 298, 5th ed.), and also by the Rev. E. Hill (Geol. Mag. 1876, p. 334); a purely graphical method by Mr. W. H. Dalton (Geol. Mag. 1873, p. 332); and an approximate method by Mr. Penning (Geol. Mag. 1876, p. 236), reproduced in his “Field Geology.” As Prof. Green has pointed out (ib. p. 377), the last-named method is equivalent to taking the angle for its tangent, and so applicable only to small angles of dip. Mr. Dalton's solution (loc. cit. p. 334) of another question, to find the effect on strata already inclined of a second tilt in a new direction, is only an approximation, and cannot be applied if the dips are considerable. It is erroneously assumed that the inclination of the strata in a direction at right angles to that of the second tilt is unaltered by the tilting.

We are indebted for the Woodcuts accompanying this note to the courtesy of Dr. John Mickleborough, whose paper on the Locomotory Appendages of Trilobites we published in our February Number, p. 80.

They serve admirably to confirm the observations of Mr. E. Billings (published in the Geological Magazine for 1871, Vol. VIII. PI. VIII. pp. 289–294) on the appendages of Asaphus platycephalus. In Fig. 1, a, a, mark the position of the anterior pair of appendages; b, b, the 10th pair; c, the articulation between the carpus and propodos; d, the articulation between the propodos and dactylus; e, the lines to the letter e mark the position of the lamelliform ? branchiae beneath the pygidium.

In Darwin's recently published work on vegetable mould, allusion is made to the fact that “In countries where the summer is long and dry, the mould in protected places must be largely increased by dust blown from other and more exposed places.” A few lines further on, however, he states that “In humid countries like Great Britain, as long as the land remains in its natural state clothed with vegetation, the mould in any one place can hardly be much increased by dust.” To this statement no exception can be taken, if we remember that it only applies to a period when the climate is humid, and to a country clothed with vegetation.